There are now a number of well established techniques for increasing the security of certain types of document. Such “security documents” include banknotes (including paper and plastic currency), bonds, legal documents, identification documents and other documents where the authenticity of the document is extremely important.
Such documents are often provided with one or more overt or covert “security features”, these including specialist inks, optically variable elements, watermarks, security threads, specialist printing techniques and particular substrate materials. These security features are used to authenticate or discriminate between documents either by manual inspection, or more often, by various automatic methods. For example, it is possible to use magnetic techniques to detect the presence of magnetic material in the security threads or printing inks. Certain printing techniques are also used which produce surface relief which can in turn also be detected automatically. Thus in many such automatic methods, various sensors are provided to generate data relating to the particular documents, the data from the security features in particular being used to distinguish between document types and between genuine and counterfeit documents. In many cases, various optical methods are used, in transmissive or reflective arrangements, including infrared and ultraviolet measurements, so as to distinguish between the different types of document in the desired manner.
One type of counterfeiting technique used particularly in banknotes is that of producing a composite note. Such notes are constructed by cutting out small pieces from genuine currency and replacing the removed pieces with counterfeit material having a similar appearance. The removed genuine pieces can then be used in the production of further counterfeit banknotes whilst the original notes from which they are extracted are also used is transactions. Such composite notes are generally formed by the gluing or taping together of pieces of counterfeit and genuine banknote material which makes their automatic detection difficult since a number of the security features within the banknotes will be from genuine notes. Other types of composite notes may also be fabricated from different notes of genuine currency, these being sometimes of different denominations. Composite banknotes are constructed to deliberately defraud either an untrained user or automatic machines which authenticate notes using techniques relating to certain features of the notes. Thus the composite notes may be targeted at particular types of automatic machines.
In addition to counterfeiting, security documents can become damaged by deliberate or accidental actions during their lifetime. It is desirable to detect such damage which may include cuts (such as incisions and tears) in the documents. This is particularly important in automated document processing apparatus, since damaged documents may cause jams and also since such damage may indicate the existence of a counterfeit document.
There is therefore an ongoing need to improve the range of methods by which automatic analysis of security documents may be performed.